Adding an ImGuiKey 'ImGuiKey_EnterSecondary' to support platforms that differentiate the enter (return key) and the numpad enter key.

examples/imgui_impl_win32.cpp

@@ -77,6 +77,7 @@ bool    ImGui_ImplWin32_Init(void* hwnd)
     io.KeyMap[ImGuiKey_Backspace] = VK_BACK;
     io.KeyMap[ImGuiKey_Space] = VK_SPACE;
     io.KeyMap[ImGuiKey_Enter] = VK_RETURN;
+    io.KeyMap[ImGuiKey_EnterSecondary] = VK_RETURN;
     io.KeyMap[ImGuiKey_Escape] = VK_ESCAPE;
     io.KeyMap[ImGuiKey_A] = 'A';
     io.KeyMap[ImGuiKey_C] = 'C';